Multimodality Imaging of a Rare Case of Retroperitoneal Fetus in Fetu: A Radiologic Diagnostic Challenge

Abstract

Introduction: Fetus in fetu is a rare congenital anomaly characterized by the presence of a malformed parasitic twin within its host, most commonly in the retroperitoneal space. Differentiation from mature teratomas is essential due to differences in malignant potential and management strategies. Case report: We report a 4-month-old male who presented with progressive abdominal distension, jaundice, and pale stools. Imaging studies revealed a large retroperitoneal mass with fetiform features. Imaging showed a retroperitoneal mass with organized skeletal structures compressing the biliary system. Surgical removal revealed a sac-like mass containing limbs and vertebral elements. Histopathology confirmed fetus in fetu without malignancy. Conclusion: Fetus in fetu should be considered in the differential diagnosis of complex retroperitoneal masses in infants. Multimodal imaging, especially the combination of ultrasonography, computed tomography, and magnetic resonance imaging, is invaluable for diagnosis, surgical planning, and distinguishing fetus in fetu from neoplastic entities.